Analog central offices generally employed 2 wire switching through the office and therefore had no 2 to 4 wire hybrids associated with the line circuits. When 4 wire trunks were used, entering or leaving the office, a 2 to 4 wire hybrid was incorporated into the trunk circuit and was designed to match against a compromise loop network which took into account both loaded and non-loaded loops as seen through the circuitry of the central office. Due to the fact, that loss was deliberately incorporated into the trunk to implement the office loss plan, and also due to the loss encountered through the 2 wire portion of its office, the appropriate match obtained through the compromise network was adequate.
Digital local offices are inherently 4 wire even on local calls, and have a greater time delay than analog offices. Both factors contribute to a higher return loss requirement, which in turn requires either better matching between the compromise network and the loop, or intentionally inserted loss on local calls, or a combination of both. Some approaches to this problem have included the insertion of a loss into local calls or have attempted to come up with a closer compromise value for the matching network. This approach sets an upper limit on the return loss that can be obtained due to the wide impedance variations between loops. In another attempt to avoid these problems a separate matching network for loaded and non-loaded loops is used. This is based on a desire to avoid inserting loss on local calls and a recognition of the fact that loaded loops tend to cluster around a particular value of impedance while non-loaded loops cluster around a different value. For this reason, each line circuit must be individually adjusted to use either the loaded or non-loaded matching network. The disadvantages of this scheme are the labor needed to strap each line circuit individually to use the proper networks, and the need for accurate office records to determine which network should be used. Office loop records are often either inaccurate or inconveniently arranged for this purpose.
Automatic measurement of the characteristics of the loop would be the ideal situation, but some problems must be solved to provide this. Time Domain Reflectometry is often used to measure the characteristics of transmission lines, but this technique does not always clearly differentiate between different types of line discontinuity (bridge tap, load coil, change of wire gauge, etc.) and also requires the application of wideband pulses to the loop. Because much of the energy of these pulses lies outside the audio band of interest, problems can develop with interference to other devices and questionable validity of the measurements, in addition to the difficulty of automatically interpreting the results.
Manual test equipment is also available which applies an audio frequency sweep to the loop and displays either the loop impedance or return loss vs. frequency. Unfortunately, not only is this equipment bulky and expensive, but it is manually operated and requires human interpretation of the test results, which do not by themselves give a clear cut indication of loaded or non-loaded loop status. Another problem with these devices is that they cannot detect the status of DC loop supervision and they are incapable of detecting when test readings are incorrect due to subscriber access at the time of the tests.
Recently, at least one paper has been published (An Improved Adaptive Electronic Circulator for Telephone Applications, H. Gazioglu, D. A. Homer, J. I. Sewell, IEEE Transactions on Communications, Vol. COM-27, No. 8, August 1979, pp. 1218-1224) that describes a telephone utilizing test tones at approximately 12 Hz, 5000 Hz, and 10,000 Hz to measure the resistance, capacitance, and rollover frequency of a telephone loop and to automatically adjust the hybrid impedance to obtain low sidetone levels. This device, however, senses only at the 3 frequencies mentioned and is inherently incapable of sensing the presence or absence of loading coils. Further, it looks at the line from the subscriber end of the loop, toward the C.0., rather than out from the C.0., and uses test tones outside the normal 200-3200 Hz pass band of telephony equipment.
Attempts have been made to perform automatic loop sensing using a single tone at a frequency above 3 kHz to determine loaded or non-loaded loop status. In addition to using out-of-band tones, this device requires an adjustment of the operating frequency depending on the loading plan used in the office. This opens an avenue for errors due to wrong settings and introduces additional complications in offices using more than one loading plan.
Other devices have attempted to use the incoming voice to drive an adaptive hybrid network while the loop is in use, but these suffer from drift and errors due to differences in the voices of different talkers and the lack of low frequencies in the incoming signals. Also these devices must be provided on a per-line-circuit basis, rather than as an infrequently used test device as is the present disclosure.